Gall adelgid

From Wikipedia, the free encyclopedia

Gall adelgid refers to adelgid species that produce galls in spruce trees, conifer species. Such galls can be produced by an infection by Adelges cooleyi (Adelges is the genus, and Adelgidae is the subfamily), a species of aphid-like woolly adelgids (order Homoptera). These infect the new buds of native spruce trees (Cooley Spruce gall adelgid) in the foothills of the Rocky Mountains in the spring. They also attack blue spruce to a lesser degree. The parasite completes two generations within the year. The parasite requires two different trees for its life cycle, the second being the Rocky Mountain Douglas-fir. They may also attack Sitka, Engelmann, or white spruce. There are many different species of adelgids that produce different gales on different spruce species.

[edit] Identification

The infection is most noticeable on Cooley Spruce in the spring, May to June, when the galls appear. This infection may be mistakingly diagnosed to as caused by worms, grubs, or even as a sex organ of the spruce. Spruce pollen, however is released from a smaller structure that lacks needles. The galls are characterized by this pineapple-like form, with a length of 0.5 cm to 8 cm depending on the growth capacity of the tree. Most galls take on a pink, red or even deep purple colour while the needles usually remain green. The segments of the new bud that have this gall form will die after the aphids leave in the summer. Once on Douglas-fir, the adelgids consume the needles but do not form galls.

Diversity of colour of galls. Can be green to red, and to deep-purple.

The end of a gall where infection was complete. The tissue has since become necrotic on this hardened remnant.

The end of a gall that had partial infection. Growth continues but it is convoluted.

[edit] Infection

In the fall, the immature female adelgid, small, globular, and wingless (1.2-1.7 mm), finds a spruce to over-winter on. In the spring when the winter thaw occurs, the female matures and lays some eggs in what resemble sacks (totalling several hundred eggs) on the branches near the developing buds. These in fact are not sacks, but individual tufts of white waxy threads that protect the eggs. The females prefer areas on the spruce where they have greater protection from the elements, especially wind. These female individuals have an obvious patch of white wax wool as a covering. Their mouthparts consist of thread-like stylets which are used to penetrate into vascular bundles for feeding.

Once hatched, the young nymphs begin feeding around the base of the needles in a new bud. The nymphs's saliva introduced into the plant trigger the changes in development of the plant, distinguished by the thickening and expansion of the basal portion of needles towards the characteristic gall form. When there are incomplete infections, the side of the bud facing the ground will be infected first. Only partially afflicted buds can support new growth after the affected tissue has died.

Large contrast box showing an infected bud (gall) beside healthy buds. Two egg tufts are visible to the left of the gall (infected bud).

Cross-section of gall in June. Nymphs are very small, but can be seen with the naked eye. The white part is a waxy excretion by the nymph.

The chamber at the base of each needle is not connected with any other chamber in the gall. This differentiates this infection from that caused by the woolly adelgid genus Pineus, where in that case the chambers are interconnected.

Cross-sectional difference between cooley spruce gall adelgid (left) and Pineus (right).¹

The nymphs are light brown when first hatched, becoming black when settled in gall chamber; they are flattened oval in shape and secrete a fringe of white wax. If a gall is opened in June, this white wax will be easily visible.

[edit] Life cycle

The life cycle of the gall adelgid requires six generations to complete, only two of which cause damage (nymph stages) and has two migration phases between the spruce and the Douglas-fir. On Douglas-fir, adults are about 0.1 cm long, oval, and light to dark brown in colour. At maturity they are completely covered with white, waxy wool and appear, from spring to fall, as stationary wool tufts on the underside of needles.¹

Non-tufted and tufted cooley spruce gall adelgids on Douglas-fir foliage. Tufts are destinguished by wool-like appearance.¹

While the chambers of the galls are closed, the nymphs actively feed and increase in size. By midsummer (August to September), the galls begin to dry out, the chambers open and winged forms of the adelgids emerge. These winged adult females have dark red-brown colour, with a heavily sclerotized thorax.³,⁴

Alate stage emerged from gall. Whitney Cranshaw, Colorado State University.

These leave the original tree and most migrate to Douglas-fir trees. The abandoned galls continue to dry out and harden as the plant tissue dies. These dead galls are remenants that are never used again. The gall portion of partial infections dies, while the uninfected segment can continue growth, resulting in curved and convoluted shapes.

Egg tufts on needles of Douglas-fir.³

Close up of an egg cluster, and the waxy strands, of the Cooley spruce gall adelgid on a needle of Douglas-fir.⁶

On Douglas-fir, eggs are laid on the needles and several generations of woolly adelgids are produced. Yellow spots and bent needles result from feeding damage.² The needles of light to moderately infested trees exhibit chlorotic mottling where individual adults have fed. Attacked needles may also be twisted. Severely infested foliage may be completely chlorotic and drop prematurely.¹ Late in the summer, some of the woolly adelgids develop wings and fly back to spruce to deposit eggs, which produce the overwintering population. Others are wingless and remain on Douglas-Fir trees, where they produce other overwintering forms.²

[edit] Predation and control

Natural predators, such as ladybirds/ladybugs, hoverfly and lacewing larvae, spiders and mites do reduce adelgid and aphid populations to some degree.⁴

Spraying againsts these adelgids with chemicals is possible, and can be done in either the fall or the early spring. Foliar treatments of carbaryl (Sevin) and permethrin have been most effective in Colorado State University trials. Horticultural oils, which have also been very effective, can cause temporary discoloration of spruce needles.²

[edit] Conifer aphids

Young trees that are large enough or having enough shade are most vulnerable to infection by aphids, such as Adelges cooleyi. Mature trees are more resistant.

The most commonly occurring aphids locally, in the Canadian Rocky Mountains, are several species of giant conifer aphids of the genus Cinara, and the Cooley spruce gall aphid Adelges cooleyi. Other aphids that have been pests are the green spruce aphid, Elatobium abietinum; the primitive woolly aphid, Mindarus obliquus; and two conifer root aphids, Pachypappa tremulae and Prociphilus xyloster. The balsam woolly aphid, Adelges piceae (also known as the balsam woolly adelgid), although a concern because of quarantine regulations, has not been reported at any local nurseries.⁵

The coniferous root aphid, Pachypappa tremulae, previously called Rhizomaria piceae, has been a pest at several British Columbia nurseries, particularly near Prince George. This aphid has taken advantage of potted seedlings.⁵

The balsam woolly aphid has become established in British Columbia and can seriously damage all true firs. Quarantines have to be respected to prevent its spread.

[edit] See also

• Hemlock woolly adelgid: another disease caused by a woolly adelgid species.
• Balsam wooly adelgid: another species of woolly adelgid.

More on the invasive woolly aphid species:

• Forest Pests. (2005). Balsam woolly aphid Adelges piceae (Ratzeburg). (World Wide Web page). Available: http://www.forestpests.org/southern/balsamwoollyadelgid.html (Dec. 1, 2005)

A University of Minnesota Pamphlet with image of mature woolly adelgid female: [1] and related pests

[edit] References

• Note 1: Ebata, Tim. March 12, 2002. Cooley Spruce Gall adelgid - Pest Guide BC Ministry of Forests, Victoria, BC, Canada. Part of:[2]

• Note 2: Cranshaw, W.S. June 23, 2005. Cooley Spruce Galls Colorado State University.

• Note 3: UBC Faculty of Forestry, Vancouver, B.C., Canada. Insects Studied in Forestry 308

• Note 4: Cooley spruce gall aphid. 1977. Wood, C. Government of Canada. Department of the Environment. Canadian Forest Service, Pacific Forest Research Centre, Victoria, BC. Forest Pest Leaflet 06. 4 p. PDF form

• Note 5: Canadian Forestry Service,2002-12-23. Online publication about Aphids

• Note 6: Childs, R. University of Massachusetts Amherst, Massachusetts, 01003. Cooley Spruce Gall Adelgid - Landscape Nursery and Urban Forestry - UMass Extension